```rslt ← part (redn ##.pred) rarg             ⍝ Partitioned reduction.

Suggested  by Michael Baas, pred returns a partitioned reduction of its argument
array. [part] is a vector of partition sizes, such that (+/part)=¯1↑⍴rarg.

Ways to code this operator include:

pred←{⍺⍺/¨(∊⍺↑¨1)⊂⍵}                            ⍝ --3   SL
pred←{(⎕io ⎕ml)←1 3 ⋄ ⍺⍺/¨(⍺/⍳⍴⍺)⊂⍵}            ⍝ 1-3   VMJ/MJ
pred←{(⎕io ⎕ml)←1 3 ⋄ ⍺⍺/⊃(⍺/⍳⍴⍺)⊂⍵}            ⍝ 12-   GQ
pred←{res←(⍴⍺)⍴⍺⍺/⍬ ⋄ res[⍺/⍳⊃⍴⍺]⍺⍺←⍵ ⋄ res}    ⍝ -2-   BT

pred←{⎕ml←3 ⋄  ⍺⍺ nz/⊃(⍺/⍳⍴⍺)⊂⍵}                ⍝ 12-   SM
pred←{⎕io←0 ⋄ (<⍀(+\⍺)∘.>⍳⍴⍵)⍺⍺ nz.×⍵}          ⍝ -2-   SM
pred←{⎕io←0 ⋄ ⍉(<⍀(+\⍺)∘.>⍳¯1↑⍴⍵)⍺⍺ nz.×⍉⍵}     ⍝ 12-   SM, where:
nz←{0≠⍺×⍵: ⍺ ⍺⍺ ⍵ ⋄ ⍺=0:⍵ ⋄ ⍺}                ⍝ auxiliary op.

Note:

1. Works for matrix (and higher rank) rarg.
2. "Each-less" solution.
3. Unrestricted operand function (eg: ',').

Examples:

2 3 3 2 +pred ⍳10
3 12 21 19

1 2 1 ×pred 2 3 4⍴⍳24
1   6  4
5  42  8
9 110 12

13 210 16
17 342 20
21 506 24

1 2 1,pred 3 4⍴⍳12
┌─┬─────┬──┐
│1│2 3  │4 │
├─┼─────┼──┤
│5│6 7  │8 │
├─┼─────┼──┤
│9│10 11│12│
└─┴─────┴──┘